It is produced on a large scale and is primarily used in the production of polymers, but also sees use in food processing, and as a process fluid in low temperature heat exchange applications. In the European Union, it has the E-number E1520 for food applications.

The compound is sometimes called ?-propylene glycol to distinguish it from the isomer propane-1,3-diol (?-propylene glycol).

Structure and properties

Propylene glycol is a clear, colorless and hygroscopic liquid. Propylene glycol contains an asymmetrical carbon atom, so it exists in two enantiomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.[3]

The freezing point of water is depressed when mixed with propylene glycol owing to the effects of dissolution of a solute in a solvent (freezing-point depression). In general, glycols are non-corrosive, have very low volatility and very low toxicity; however, the closely related ethylene glycol (a key ingredient in antifreeze) is toxic to humans and to many animals[4].

Production

Industrial

Industrially, propylene glycol is produced from propylene oxide[5] (for food-grade use), and global capacity in 1990 was 900,000 tonnes per year.[6] Different manufacturers use either non-catalytic high-temperature process at 200 °C (392 °F) to 220 °C (428 °F), or a catalytic method, which proceeds at 150 °C (302 °F) to 180 °C (356 °F) in the presence of ion exchange resin or a small amount of sulfuric acid or alkali.

Final products contain 20% propylene glycol, 1.5% of dipropylene glycol and small amounts of other polypropylene glycols.[7] Further purification produces finished industrial grade or USP/JP/EP/BP grade propylene glycol that is typically 99.5% or greater. Propylene glycol can also be converted from glycerol, a biodiesel byproduct. This starting material is usually reserved for industrial use because of the noticeable odor and taste that accompanies the final product.

Like ethylene glycol, propylene glycol is able to lower the freezing point of water, and so it is used as aircraft de-icing fluid.[6][15] Water-propylene glycol mixtures dyed pink to indicate the mixture is relatively nontoxic are sold under the name of RV or marine antifreeze. Propylene glycol is frequently used as a substitute for ethylene glycol in low toxicity, environmentally friendly automotive antifreeze. It is also used to winterize the plumbing systems in vacant structures.[16] The eutectic composition/temperature is 60:40 propylene glycol:water/-60 °C.[17][18] The -50 °F/-45 °C commercial product is, however, water rich; a typical formulation is 40:60.[19]

Propylene glycol is used in veterinary medicine as an oral treatment for hyperketonaemia in ruminants. Glucose, which can be used in non-ruminants for this purpose, is not effective due to its consumption by the resident microbes of the rumen. Propylene glycol is partially metabolized in the rumen to propionate which can be used as an energy source. The remainder is absorbed into the bloodstream and used by the liver for gluconeogenesis.[20]

Safety in humans

Oral administration

The acute oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health damage in humans; propylene glycol is metabolized in the human body into pyruvic acid (a normal part of the glucose-metabolism process, readily converted to energy), acetic acid (handled by ethanol-metabolism), lactic acid (a normal acid generally abundant during digestion),[21] and propionaldehyde (a potentially hazardous substance).[22][23][24]

Serious toxicity generally occurs at plasma concentrations over 4 g/L, which requires extremely high intake over a relatively short period of time, or when used as a vehicle for drugs or vitamins given intravenously or orally.[25] It would be nearly impossible to reach toxic levels by consuming foods or supplements, which contain at most 1 g/kg of PG, except for alcoholic beverages in the US which are allowed 5 percent = 50g/kg.[26] Cases of propylene glycol poisoning are usually related to either inappropriate intravenous administration or accidental ingestion of large quantities by children.[27]

The potential for long-term oral toxicity is also low. In one study, in 1972, 12 rats were provided with feed containing as much as 5% PG over a period of 2 years, and showed no apparent ill effects; no data on offspring were offered.[28] Because of its low chronic oral toxicity, propylene glycol was classified by the U. S. Food and Drug Administration as "generally recognized as safe" (GRAS) for use as a direct food additive, including frozen foods such as ice cream and frozen desserts.[26][29] The GRAS designation is specific to its use in food, and does not apply to other uses.[30]

Skin, eye and inhalation contact

Prolonged contact with propylene glycol is essentially non-irritating to the skin.[31] Undiluted propylene glycol is minimally irritating to the eye, producing slight transient conjunctivitis; the eye recovers after the exposure is removed. Exposure to mists may cause eye irritation, as well as upper respiratory tract irritation. Inhalation of propylene glycol vapors appears to present no significant hazard in ordinary applications.[32] However, limited human experience indicates that inhalation of propylene glycol mists could be irritating to some individuals.[33] It is therefore recommended that propylene glycol not be used in applications where inhalation exposure or human eye contact with the spray mists of these materials is likely, such as simulated fogs for theatrical productions, or antifreeze solutions for emergency eye wash stations.[34]

According to a 2010 study by Karlstad University, the concentrations of PGEs (counted as the sum of propylene glycol and glycol ethers) in indoor air, particularly bedroom air, has been linked to increased risk of developing numerous respiratory and immune disorders in children, including asthma, hay fever, eczema, and allergies, with increased risk ranging from 50% to 180%. This concentration has been linked to use of water-based paints and water-based system cleansers. However, the study authors write that glycol ethers and not propylene glycol are the likely culprit.[35][36][37]

Animals

Propylene glycol is an approved food additive for dog and sugar glider food under the category of animal feed and is generally recognized as safe for dogs,[44] with an LD50 of 9 mL/kg. The LD50 is higher for most laboratory animals (20 mL/kg).[45] However, it is prohibited for use in food for cats due to links to Heinz bodyanemia.[46]

Allergic reaction

Individuals who cannot tolerate propylene glycol experience inflamed dry skin in the facial area, or small red dots on the body. Investigators believe that the incidence of allergic contact dermatitis to propylene glycol may be greater than 2% in patients with eczema or fungal infections, which are very common in countries with lesser sun exposure, and lower than normal vitamin D balances. Therefore, propylene glycol allergy is more common in those countries.[47]

Environmental

Propylene glycol is known to exert high levels of biochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol.[48]:2-23

Sufficient dissolved oxygen levels in surface waters are critical for the survival of fish, macroinvertebrates, and other aquatic organisms. If oxygen concentrations drop below a minimum level, organisms emigrate, if able and possible, to areas with higher oxygen levels or eventually die. This effect can drastically reduce the amount of usable aquatic habitat. Reductions in DO levels can reduce or eliminate bottom feeder populations, create conditions that favor a change in a community's species profile, or alter critical food-web interactions.[48]:2-30

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